This invention relates to a sampling frequency converter for converting an input sample train to an output sample train which is not synchronous with the input sample train and, more particularly, to a sampling frequency converter designed to have an improved conversion accuracy with a simple structure.
Various sampling frequencies such as 32 kHz, 44.1 kHz and 48 kHz are used in digital devices such as digital audio devices. For connecting devices which use different sampling frequencies together, it is necessary to convert a sample train outputted from a signal transmitting device to a sample train of a sampling frequency which is used in a receiving device. In a case, for example, where master recording data which has been recorded in a studio with a sampling frequency of 48 kHz is dubbed for recording on a Compact Disc, the sampling frequency must be converted to a frequency of 44.1 kHz.
Similarly, for connecting two digital devices which have the same sampling frequency but are driven by master clocks which are different from each other, it is necessary to synchronize a sample train outputted from a transmitting device with a sampling frequency of a receiving device and this requires a kind of sampling frequency conversion. In a case, for example, data is reproduced from a Compact Disc having a sampling frequency of 44.1 kHz and is dubbed by a digital recording device having the same sampling frequency of 44.1 kHz but is driven by a master clock which is different from one for the Compact Disc, sampling frequency conversion is required for synchronizing a digital signal reproduced from the Compact Disc with the sampling frequency of the digital recording device.
For converting a sampling frequency, there is a prior art method according to which an input sample train is digital-to-analog converted to convert a digital signal to an analog signal and this analog signal is analog-to-digital converted with a sampling frequency of an output sample train.
There is another prior art method fox converting a sampling frequency according to which an input sample train is oversampled to a sampling frequency which is the least common multiple of the input sample train and an output sample train and samples which constitute the output sample train are extracted from the sampling frequency which is the result of the oversampling.
The former prior art method of sampling frequency conversion has the disadvantage that it produces a large quantizing error due to double quantizing occurring in the digital-to-analog conversion and the subsequent analog-to-digital conversion with resulting generation of a large distortion in an acoustic signal.
The latter prior art method of sampling frequency conversion according to which an input sample train is oversampled has the disadvantage that a large scale of device is required except for a case where the ratio of an input sample train and an output sample train is a simple integer ratio.
There has also been proposed a method which is an improvement of the method using oversampling. According to this improved method, an input sample train is oversampled with a multiple which is chosen in a range which will not result in requirement for a large scale device and an interval between samples produced by this oversampling is interpolated with a linear interpolation to provide an output sample train. This proposed method, however, is disadvantageous in that the conversion accuracy is not so good because this method employs the linear interpolation with a result that distortion in a produced acoustic signal becomes relatively large.
It is, therefore, an object of the invention to overcome the above described disadvantages of the prior art sampling frequency converters and provide a sampling frequency converter which can achieve a high conversion accuracy with a relatively simple structure.